In typical usage of content data such as music, the data stored in a primary recording medium such as an HDD (hard disk drive) of a personal computer is transferred to another recording medium used as a secondary recording medium so as to allow the data reproduced from the secondary recording medium to be enjoyed. It is to be noted that the content data includes musical data, video data, game data and computer software, which are provided mainly for distribution, transfer and utilization purposes.
In this case, the HDD employed in the personal  computer is used for storing content data such as music reproduced from a package recording medium such as a CD-DA (Compact Disc Digital Audio) and a DVD (Digital Versatile Disc) or used for storing content data downloaded from an external musical server or the like by way of a communication network to the personal computer, which is connected to the network. Then, the user connects the personal computer to a recording apparatus for recording data onto the secondary recording medium, and copies or moves content data from the HDD to the secondary recording medium. In order to enjoy the content data, a reproduction apparatus is used to reproduce the data from the secondary recording medium.
Conceivable examples of the secondary recording medium are a memory card employing a semiconductor memory such as a flash memory, a mini disc used as a magneto-optical disc, a CD-R (CD-Recordable), a CD-RW (CD-Rewritable), a DVD-RAM, a DVD-R and a DVD-RW.
As a recording apparatus and a reproduction apparatus for the secondary recording medium, respectively, a recorder and a player for these recording mediums including the semiconductor memory, the mini disc, the CD-R, the CD-RW, the DVD-RAM, the DVD-R and the DVD-RW have been becoming very popular. The recorder and the  player are designed into a variety of implementations such as a stationary recording/reproduction apparatus and a portable recording/reproduction apparatus. The user can thus record and reproduce content data by using an implementation of the recorder and the player that is a favorite with the user or matches an apparatus already owned by the user.
It is to be noted that, when considering such usage of content data, for example, protection of copyrights of the content data must be taken into account. Assume for example that the user obtains content data by using a service to distribute the content data or by purchasing a package medium containing the content data and, after storing the content data into an HDD, the user is allowed to copy the content data to secondary recording mediums without any limitation. In this case, such usage of the content data results in a condition in which the owner of the copyright is not properly protected. In order to solve this problem, there have been proposed a variety of agreements and technologies for assuring protection of copyrights in handling content data as digital data. One of the agreements is a standard called an SDMI (Secure Digital Music Initiative).
A data path prescribed in the SDMI standard will be  described later. At any rate, a content stored in an HDD employed in a personal computer as a primary recording medium can be properly transferred to or recorded onto a secondary recording medium after protection of a copyright and benefits of general users are taken into consideration. The benefits include a right to privately copy the content. Examples of the content stored in the HDD include a network content and a disc content. The network content is content data distributed by an external server to the personal computer by way of typically a network to be stored in the HDD. On the other hand, a disc content is read out from a package recording medium to be stored in the HDD. As described above, examples of the package recording medium are a CD-DA and a DVD. The package recording medium is mounted on a disc drive for reproducing the disc content. Typically, the disc drive is embedded in the personal computer or connected to the personal computer. Examples of the disc drive include a CD-ROM drive.
By the way, when content data is transferred in a copy operation from a primary recording medium such as an HDD to a secondary recording medium such as a mini disc or a memory card, measures are taken to satisfy protection of both a copyright and a private copying  right.
In order to satisfy such protection, data is transferred from a secondary recording medium conforming to the SDMI standard as follows.
A secondary recording medium conforming to the SDMI standard is assumed to be a recording medium including a memory card employing a semiconductor memory such as a flash memory in conformity with the SDMI standard. Such a secondary recording medium is used for storing a content in an encrypted state. In a primary recording medium like an HDD, for example, a content conforming to the SDMI standard is stored in an encrypted state so that such a content will be copied to a secondary recording medium also in the encrypted state as it is.
It is needless to say that a reproduction apparatus provided for such a secondary recording medium has a decryption function allowing content data copied to the secondary recording medium in an encrypted state to be reproduced.
In a secondary recording medium conforming to the SDMI standard, a recording format includes an area for recording a content ID used as an identifier for identifying each piece of content data.
A content ID is generated for each piece of content  data stored in the primary recording medium such as an HDD employed in an apparatus for the primary recording medium and is stored along with the piece of content data. When content data is copied to a secondary recording medium, a content ID identifying the content data is also stored in the secondary recording medium.
Content IDs are used for managing content rights in primary and secondary recording mediums. A content right of a content in a primary recording medium is a right to transfer the content from the primary recording medium to a secondary recording medium in an operation to copy the content to the secondary recording medium. On the other hand, a content right of a content in a secondary recording medium is a right to reproduce the content from the secondary recording medium.
It is to be noted that, in the following description, a transfer of content data (a transfer of a right) from a primary recording medium to a secondary recording medium is referred to as a check-out. On the other hand, a transfer of content data (in actuality, a transfer of a right only) from a secondary recording medium to a primary recording medium is referred to as a check-in.
In accordance with the SDMI standard, transfer  usage rules are established for check-outs and check-ins.
For example, for a piece of content data, only up to three check-outs from a primary recording medium to a secondary recording medium are allowed. Thus, a transfer right allows the piece of content data to be transferred up to three times.
In a check-out, a right is also transferred from a primary recording medium to a secondary recording medium. Thus, the transfer right in the primary recording medium now allows the piece of data to be transferred only up to two times. On the other hand, a reproduction right is given to the secondary recording medium.
In a check-in, on the other hand, a right is returned from a secondary recording medium to a primary recording medium. Thus, the secondary recording medium loses a reproduction right while one transferred right is restored to the primary recording medium.
Such check-outs and such check-ins are managed for each piece of content data by using a content ID for identifying the piece of content data.
In addition, in a check-out, a piece of content data and a content ID for identifying the piece of content data are recorded onto a secondary recording medium. The secondary recording medium is given a  reproduction right to reproduce the piece of content data. On the other hand, the primary recording medium is considered to have transferred one content ID and lost one transfer right in accordance with the usage rules.
In a check-in, on the other hand, no content data is actually returned. Instead, the content data is merely erased from the secondary recording medium, and the content ID is returned to the primary recording medium to increment the number of transfer rights prescribed by the usage rules by one. The secondary recording medium loses the reproduction right to reproduce the content data.
As described above, content data is copied to a secondary recording medium conforming to the SDMI standard in an encrypted state to be stored into the recording medium also in an encrypted state, and content rights are managed in the event of a check-out or a check-in in order to prevent content data to be copied an unlimited number of times and to protect copyrights as well as to assure the private-copy right of the user at the same time.
It is to be noted that content data downloaded from typically an external server to an HDD serving as a primary recording medium is stored in the HDD in a state of being encrypted by using a content key CK. 
In this specification, content data stored in the HDD is assumed to have been obtained as a result of compressing original content data by adoption of an ATRAC3 technique or another compression technique and encrypting the compressed data A3D by using the content key CK. In this specification, notation E (x, y) denotes encrypted data obtained as a result of encrypting data y by using a key x.
On the other hand, notation D {x, E (x, y)} denoted decrypted data obtained as a result of decrypting encrypted data E (x, y) by using the key x.
Thus, the content data obtained as a result of compressing original content data and encrypting the compressed data A3D by using the content key CK can be expressed by notation:                E (CK, A3D)On the other hand, decrypted data obtained as a result of the decrypting encrypted data E (CK, A3D) by using the key CK is expressed by notation:        D {CK, E (CK, A3D)}        
In addition to the encrypted content data E (CK, A3D), the HDD serving as the primary recording medium is also used for storing E (KR, CK), which is the content key CK in a state of being encrypted by using a root key  KR. Thus, in the case of encrypted content data E (CK, A3D) downloaded from an external server, for example, the encrypted content key E (KR, CK) is also downloaded from the server.
In this case, in a check-out of the encrypted content data E (CK, A3D) to a secondary recording medium, the encrypted content data E (CK, A3D) and the encrypted content key E (KR, CK) need to be transferred from the HDD serving as the primary recording medium to the secondary recording medium.
The apparatus for the secondary recording medium holds the root key KR for decrypting the encrypted content key E (KR, CK) to produce the original content key CK. The content CK is then used for decrypting the encrypted content data E (CK, A3D) to produce the original content data A3D.
In accordance to the will of the copyright owner or for a variety of reasons, however, the root key KR can be changed. That is to say, a root key KR can be set for each piece of content data. In addition, there is provided a function for imposing restrictions on targets of content distribution by processing of the root key KR. This function will be described later concretely.
Thus, in some cases, data called an EKB (Enabling  Key Block) is distributed. In addition, in some cases, a regular terminal for receiving transferred content data adopts a technique for confirming a root key by using an EKB. That is to say, an EKB is distributed from a server to be stored in an HDD along with encrypted content data and an encrypted content key.
Consider a case in which a mini disc (or a magneto-optical disc), which has been becoming very popular, is used as a secondary recording medium in a mini-disc recording apparatus conforming to the SDMI standard. In this case, encrypted content data E (CK, A3D) transferred to the mini disc in a check-out is stored in the mini disc in an encrypted state as it is.
Then, in a reproduction operation, the mini-disc recording apparatus conforming to the SDMI standard decrypts the encrypted content data E (CK, A3D) to produce the content data-D {CK, E (CK, A3D)}=A3D, which is the content data compressed by adoption of the ATRAC3 compression technique. Then, the mini-disc recording apparatus carries out a predetermined decoding process on the compressed content data A3D to output reproduced data such as music.
In a mini disc used in an ordinary mini-disc system, which has been becoming popular too, on the other hand,  data is stored not in an encrypted state. Thus, as a mini-disc reproduction apparatus, the mini-disc system naturally does not have a decryption function.
As a result, content data recorded in a mini disc by the mini-disc recording apparatus conforming to the SDMI standard cannot be reproduced by most of mini-disc players, which do not conform to the SDMI standard. That is to say, the content data recorded in a mini disc by the mini-disc recording apparatus conforming to the SDMI standard is not reproduction-compatible with the mini-disc players.
This means that correct use of an SDMI content purchased by the general user is limited and the value of the service to provide an SDMI content to the ordinary user as well as the level of user satisfaction with the service are hence reduced considerably.
In order to solve the problem described above, in a recording operation to copy an SDMI content to a secondary recording medium such as a mini disc mounted on a mini-disc recording apparatus not conforming to the SDMI standard, the SDMI content is decrypted so that the content can be stored in the mini disc in an unencrypted state as it is.
If such a copy operation can be carried out,  however, it is possible to copy content data with ease. Such a copy operation also leaves room for possible illegal copies, resulting in a fear of impossibility to implement copyright protection, which is the original objective of the SDMI standard.
In order to solve the problem described above, as a method of transferring content data, the applicant of a patent for the present invention has proposed a content transfer technique described as follows.
In an operation to transfer content data, a data transfer apparatus serving as a primary-recording-medium apparatus authenticates a data-recording apparatus used as a secondary-recording-medium apparatus to serve as a transfer destination. If the result of the authentication is OK, the transfer of the content data is allowed on the condition that the content provider (such as the copyright owner) approves the transfer. Then, the content data is transmitted in an encrypted state through a transmission line and the encrypted content data is decrypted before being stored in a secondary recording medium. In addition, rights are managed for check-outs and check-ins.
In this way, since an operation to copy and record content data in an unencrypted state is permitted, more  convenience is offered to the user without losing the function to protect a copyright.
By adoption of such a technique in a system including a personal computer used as the data transfer apparatus and a mini-disc recorder used as a data-recording apparatus, a transfer of a content from a primary recording medium implemented by an HDD employed in the personal computer to a secondary recording medium implemented by a mini disc mounted on the mini-disc recorder is actually well suitable for both the content provider and the user.
By using a medium such as a mini disc, which has been becoming popular in general, as the destination of a check-out described above, the usefulness of the mini disc in practical use is increased. However, there are cases in which the data transfer apparatus such as a personal computer may not be capable of recognizing the condition or other attributes of a medium adopting its own unique data management method. An example of the medium adopting its own unique data management method is a mini disc.
For instance, content data recorded in a memory card given as an example of a medium conforming to the SDMI standard is managed by using a FAT so that the  memory card is well compatible with a personal computer. That is to say, the personal computer serving as a data transfer apparatus is capable of obtaining information on the state of the memory card connected as a removable medium used as a destination in a check-out by merely reading out a FAT from the card. The state of the memory card typically includes the storage capacity of the card and the number of contents recorded on the card.
Thus, when it is desired to transfer out certain content data to the memory card in a check-out, it is possible to form a judgment as to whether or not the content data can be transferred out to the memory card on the basis of the storage capacity of the card and other information.
In the case of a medium developed as a medium adopting its own unique data management method for typically audio-data applications as is the case with a mini disc, however, the personal computer is not capable of directly obtaining information on the state of the medium so that the judgment cannot be formed in a check-out or other operations.
Thus, when the use of a mini disc or another medium as a secondary recording medium at the destination of a check-out is assumed, the personal computer serving as a  data transfer apparatus is required to have a method allowing the state of the mini disc or the other medium to be recognized.